1. Field of the Invention
The present invention relates to a slot antenna arranged at an upper side of a ground conducting plate, and particularly to a slot antenna having a high gain in the zenith direction.
2. Description of the Related Art
FIG. 6 is a plan view showing a conventional general slot antenna. A slot 2 extending in a straight line is provided in a conducting member 1 made of a metal plate or a metal foil, and a feeding pin 3 for supplying a high frequency power to the conducting member 1 is provided perpendicular to a predetermined feeding point. The feeding pin 3 is connected to an amplifying circuit or a filter circuit (not shown), and the power is fed by the feeding pin 3 to excite the slot 2. Also, since the electric field traversing the slot 2 in the width direction is generated upon the excitation, the horizontally polarized wave is radiated toward the just upper side (the zenith direction) or is obliquely radiated toward the upper side, and thus the radio wave signal transmitted in these directions can be received. In addition, as a feeding method, there is a structure which the feeding point is arranged at the vicinity of both ends of the slot 2 in the width direction (for example, see Japanese Unexamined Patent Application Publication No. 2003-218629 (page 2, FIG. 5)) or a structure which a feed line perpendicular to the slot 2 is provided in the lower side of the conducting member 1.
Since such a slot antenna can be cheaply manufactured and can be easily miniaturized, it is suitable to an antenna device for a vehicle. In other words, when the conducting member 1 having the slot 2 is provided on the top surface of the dielectric substrate and an electronic circuit such as an amplifying circuit is mounted on the bottom surface of the dielectric substrate, a cheap small-sized antenna device is obtained. In addition, if an upper plate portion of a shield case accommodating the circuit substrate is used as the conducting member 1 and the slot 2 is provided on the upper plate portion, a very cheap antenna device can be obtained.
However, in the case in which the slot antenna is applied to the antenna device for a vehicle, there are many cases that the dielectric substrate or the shield case is provided on a relatively large ground conducting plate. However, as shown in FIG. 7, if the ground conducting plate 4 extends toward the outside of the conducting member 1 having the slot 2, the reverse electric field Eb is induced between the conducting member 1 and the ground conducting plate 4 upon the excitation of the slot 2. Thereby, the original electric field Ea traversing the slot 2 in the width direction is apt to be canceled by the reverse electric field Eb. At the result, the radiation toward the inclined upper side due to the reverse electric field Eb becomes strong, but the radiation toward the zenith direction becomes weak. Therefore, for example, in the case in which it is applied to the antenna device for ETO (Electronic Toll Collection), it is difficult to obtain a desired sensitivity.